This invention concerns the domain of data transmission in a network of apparatuses linked to a broadcasting center and without any permanent return connection.
In a Pay-TV network, it frequently occurs that the users' apparatuses receive control signals from a management center, these signals allowing the management of the access security to the broadcasted data.
Complex mechanisms have been developed so that many individual functions can be accessed without needing a connection to a management center.
Among these mechanisms, one can cite on demand access to contents thanks to the presence of a credit to be freely allocated to the event of one's choice.
“Contents” is understood as an information service concerning the stock-exchange, the weather forecast, general-interest television, a sporting event, etc. These contents can be broadcasted on the units of certain users such as a Pay-TV decoder, a computer or even a mobile phone, a palmtop, a PDA, a radio, a television or a multimedia terminal.
The digital flow is enciphered in order to be able to control the use thereof and to define certain conditions for such use. This encryption is made thanks to control words, which are changed at regular intervals (between 5 and 30 seconds) in order to deter any attack aiming to find such a control word.
The user unit includes a security module, generally removable, which manages the security operations such as the verification of the rights and the decryption of the necessary keys (control words) for the decoder to access the transmitted data.
The accounting of use of such contents is today based on the principle of the subscription or event purchasing. Subscription allows the definition of a right associated with one or more broadcasting channels and allows the user to access these channels in clear if the right exists in his security unit.
At the same time, it is possible to define rights pertaining to a particular content, such as a film or a football match. The user can acquire this right (purchase for example) and this content will be specifically managed by this right. This method is known under the term impulse purchasing (pay-per-view PPV).
During the decryption of the control words, it will be verified if a right associated with the access conditions is present in the security module.
Whenever the comparison is positive the control word is returned in clear to the user unit.
Some users' units have a back channel to communicate the user's choice and therefore to acquire the rights pertaining to a particular choice. This channel is generally a modem connected to a telephone line or to a network. Due to the reluctance of certain people to any form of supervision on the actual consumption of the contents, the cable is simply not linked to the phone line socket and the data is not transmitted to the management center. It may be that this connection is not carried out for practical reasons, for example because the phone line socket is too far from the TV set.
At the time of establishing a security module for a new client, a credit is usually attributed to him allowing him to test the practicality of the impulse buy. This credit is not invoiced to the client, because it may never be used.
In normal operation conditions, the user buys one or two films with this credit and connects his user unit for reloading. At this moment, the management center establishes the invoice for the two films purchased and reloads the module with the amount that has previously been agreed upon with the user.
The management center cannot invoice the consumption carried out on this initial credit until the user connects his unit to a network. In this way, the total amount of the non-invoiced credits on all the security units can be considerable.
Actions have been taken such as the proposal of games to encourage, by the distribution of a price, the users to connect their unit.
The problem definitively arises when a change of security module is carried out, in general for technological evolution reasons. The operator is satisfied when the user carries out this exchange on time and it is difficult to oblige the user to return his old card with the aim of invoicing services. In this way, the credit use accounting data contained in the previous module is considered as lost.